Presta valve in Schrader hole?

Can it be done? I have a spare rear wheel that takes the Schrader (car) valve, but I'd like to use it with a Presta tube. Bike uses Presta valve on the front wheel and I don't want to end up having to stock / carry round 2 tube types).

Currently, the Presta rattles around in the hole when offered up, and when I tried inflating it a little, the valve protrudes through the hole so that it won't seat, even when the lock ring is wound right down to the base of the valve.

Is there an adapter that will reduce a Schrader hole to take a Presta valve, or is it a no-go?

Cheers steve - I'd wondered about putting a spare lock ring on the valve and using one each side of the hole in the rim. It might hold the valve steady - but do you think it might cause problems with the tube at 116psi?

I, too, was told by a LBS that a sort of grommet was available, though they didn't have any in stock, and in the end I used the 'lock ring' dodge as mentioned above. I was warned, however, that if a rim is drilled for Schrader it may not be able to withstand typical presta valve pressures, adapter or no adapter. This may be a matter of opinion, still, I wouldn't trust the setup over about 90psi.

I do this with all my rims, so I can use either tube. I have some rubber grommets that came from Gearshift, it doesnt look like they're available now.
My latest Mavic rim (717?) came like this with a plastic insert. I got it from Holts, in Derby, so it'd be worth ringing to see if they have any spares.